Various means for prolonging the life of cut flowers have been tried. For example the typical methods are as follows: 1) a flower is cut in water to improve water raising, 2) water is refreshed properly to avoid contact with rotten water and 3) the cut end of the stalk of a cut flower is burnt. However, in general, the methods of cutting a flower in water to improve water raising and of burning the cut end have very little effect and frequent refreshment of water is troublesome giving no high expectation of improving the life-prolonging effect.
On the other hand, a number of various chemicals called life-prolonging agents for cut flowers have been marketed from flower shops and gardening shops. Their life-prolonging effects and activities on cut flowers are different from each other. The main functions of the life-prolonging agent can be classified as (1) sterilization and prevention of growth of bacteria, (2) purification and clarification of water and (3) nutrition.
Chlorine-containing chemicals are used as the life-prolonging agents (1). Though such chlorine chemicals are excellent in sterilizing effect, most of them are volatile and thus exhausted in use and dispersed in air which cause them to lose their activities and to lose the life-prolonging effect on cut flowers. Therefore, a large quantity of the life-prolonging agent is required to maintain the effect. Also, chlorine chemicals may provide the potential for the formation of toxic chlorine gas. The quantities of chlorine-containing chemicals are inevitably restricted in order to prevent the chlorine odor. As a result, the chemical is eluted little by little for use. However, such a method has a disadvantage in that it cannot provide satisfactory life-prolonging effect. Such sterilizers include, for example, sodium dichloroisocyanurate, hypochlorites and chlorites.
As the life-prolonging agents (2), water-soluble inorganic aluminum compounds are generally used. They function to aggregate impurities and dust contained in water, to keep the water clear in which cut flowers are placed, to eliminate clogging of the ducts of the cut flowers with impurities, and to promote clear water raising to the stalks of the cut flowers. Because such a life-prolonging agent is generally odorless and nonvolatile, their effects are not eliminated as the time passes as seen in the aforementioned life-prolonging agents (1). However, the life-prolonging agents (2) are generally liquid and have a low pH and are difficult to handle. Such life-prolonging agents include, for example, polyaluminum chloride, aluminum sulfate, potassium alum and the like.
The nutritive agents (3) supply positive nutrients to cut flowers to aid the life-prolonging effect. However, they have substantially no effect on removing various bacteria and impurities present in water but rather tend to help bacteria proliferate. Further, since the nutritive agent alone has little life-prolonging effect on cut flowers, it is required to use it together with a sterilizing agent as described above. The nutritive agents are exemplified by glucose, sucrose, urea, potassium salts and the like.
Also as disclosed in U.S. Pat. No. 4,167,832, a sheet for keeping cut flowers fresh has been developed in which a life-prolonging agent layer containing chlorine sterilizers and so on is provided on one side of a substrate, while a pressure sensitive adhesive layer and a release liner are laminated on the other side of it. This sheet for keeping cut flowers fresh is used so that the release liner is peeled off and then the adhesive layer of the laminate of life-prolonging agent layer/substrate/pressure sensitive adhesive layer is wound on the stalk and is put into a vase as it is.
The surface layer of this sheet for keeping cut flowers fresh, i.e., the life-prolonging agent layer, contains a chlorine sterilizer and the chlorine sterilizing agent tends to contact with bare hands in many cases during treatment and thus it is detrimental to health. Also, when the sheet is placed in a vase together with cut flowers, almost all of the chlorine sterilizer is dissolved in water in a short period as it is exposed on the surface. When a large quantity of the chlorine sterilizer is dissolved out in such a way, chlorine odor floats in the surroundings which can badly affect the cut flowers. In addition, the chlorine sterilizer gradually loses its activity and therefore it becomes difficult to keep its life-prolonging effect for a long period when almost all of the sterilizer has been released in a short period after being placed in water. In order to offset such a fault of the sheet for keeping cut flowers fresh, U.S. Pat. No. 5,102,715 introduces a structure in which almost all parts of the periphery of the life-prolonging agent layer containing the chlorine sterilizer is covered with a water-impermeable layer and thus the chemical is dissolved out gradually through a specified part. However, this structure is also insufficient in the life-prolonging effect for cut flowers and a higher life-prolonging effect was sought.
The object of the invention is to provide a sheet for keeping cut flowers fresh which has a sufficient life-prolonging effect and can be easily handled and is very hygienic.
In order to select a life-prolonging agent optimum for the purpose, the life-prolonging effects of various life-prolonging agents for cut flowers mentioned above have been investigated. As the results, it was found that aluminum compounds, clarifiers for service water, have higher effect than other types of life-prolonging agents. Among them, aluminum sulfate was found to have an excellent life-prolonging effect on cut flowers.
As to aluminum sulfate, Japanese Laid-Open Patent Publication No. 108601 of 1990 proposes a method in which a substance prepared by adding a saccharide to aluminum sulfate is made as an aqueous solution or a powder and is placed into the water used for cut flowers.
However, aluminum sulfate is usually used in liquid form for use as a water-treating agent and requires troublesome handling. Furthermore, it is highly acidic and therefore tends to cause problems such as chemical damages by scattering. Even when used as powder, it is contacted by bare hands which is hygienically unfavorable. In any event, to use a liquid clarifier such as aluminum sulfate as a life-prolonging agent for cut flowers, the conventional methods have various difficulties in handling and hygiene and thus include difficulties in practical use at present.